136 PRACTICAL BOTANY 



line of the section : the walls, especially the outer, are much 

 thickened : externally there is a cuticle. 



2. Cortical tissue, consisting of cells having rather thick cel- 

 lulose walls (blue with chlor-zinc-iodine), and protoplasmic con- 

 tents with chlorophyll. Many cells have recently divided : this 

 is necessary to keep pace with the growth in thickness of the 

 vascular cylinder. Large intercellular spaces (resin-passages) 

 occur here and there, and are lined with small-celled epithe- 

 lium. 



Near the periphery of the cortex may be found a layer of 

 cork and a cork-cambium (compare stem of Elm, p. 54), 

 derived from cells of the cortex by their division by tangential 

 walls. The mature cells of the cork have no cell-contents : 

 their walls are coloured yellowish brown with chlor-zinc-iodine. 



3. The vascular system is here a complete ring, though it is 

 composed of separate bundles in the bud, as may be seen if 

 sections be cut from the axis of a dormant winter bud. Dis- 

 tinguish the external phloem, with its bright-looking cellulose 

 walls (blue with chlor-zinc-iodine), the internal xylem, the com- 

 ponents of which have thick dark-looking lignified walls (yellow 

 with chlor-zinc-iodine), and the misty layer of cambium between 

 them. 



Observe that the internal limit of the vascular ring is sinuous : 

 the convexities mark the position of the primary bundles : at 

 the inner limit of these will be found the protoxylem. 



4. The pith consists of parenchyma, having the same charac- 

 ters as in the bud : there are no resin passages. 



Put on a high power, and examine the cambium. Note 



i. That the cells are arranged with great regularity in radial 

 rows. 



ii. That their wa'Jls are thinner than those of the surrounding 

 tissues, and are composed of cellulose (blue with chlor-zinc- 

 iodine), 



iii. That the tangential walls are thinner than the radial. 



iv. That the cells have copious protoplasm, in which a nucleus 

 may often be recognized. 



These facts point to a repeated division of cells by tangential 

 walls. (Compare Fig. 7, A, p. 64.) 



